Hi Silvio,
idle notebooks get disconnected, after around 1 hour or so. Running notebooks are culled when they run too long (I think it's 24 hours, maybe 48). Also, if you have a running notebook and happen to close your laptop, this will usually also sever the connection to the notebook kernel.
Finally, in the past few days we had to restart the NB server because of uncontrolled resource overuse by a few users, and also due to a network switch issue.
If you have long-running queries, please submit them in async mode. These run independently of the kernel, and you can retrieve the results days later just by using the jobID that an async query call returned.
Please let us know if this was or wasn't useful, and we'll try to help more.